1. Field of the Invention
This invention relates to a gel electrolyte comprised of a non-aqueous electrolytic solution immersed in a matrix polymer and a gel electrolyte battery employing this electrolyte.
2. Description of the Prior Art
Recently, more and more importance is attached to the battery as a light source for a portable electronic equipment, such as a video camera or a notebook type personal computer. For reducing the size and the weight of the electronic equipments, there is raised a demand for a battery which is not only large in capacity but also is lightweight and space-saving. From such viewpoint, a lithium battery having a high energy density and a high output density is suited as a light source for the portable electronic equipment.
Of the lithium batteries, a wide variety of configurations, such as batteries exhibiting flexibility and a high degree of shape freedom, sheet batteries of large area and reduced thickness or card batteries of reduced area and reduced thickness, are desired. In the conventional technique of sealing battery elements comprised of a positive electrode and a negative electrode and an electrolytic solution in the interior of a metal outer can, it is difficult to fabricate the batteries of these various configurations. On the other hand, due to the use of the electrolytic solution, the manufacturing process is complicated, or measures need to be taken against leakage of the electrolytic solution.
In order to overcome these problems, there have been proposed batteries employing a solid electrolyte employing in turn an electrically conductive organic high polymer or inorganic ceramics or those employing a gel-like solid electrolyte comprised of an electrolytic solution immersed in a matrix polymer, referred to below as a gel electrolyte. In these batteries, in which the electrolyte is immobilized, it is possible to maintain contact between the electrode and the electrolyte. Thus, in these batteries, there is no necessity of sealing the electrolytic solution in the outer metallic can, while it is possible to fabricate the battery to a small thickness using a film-like exterior material.
As the external sheathing material for the battery, employing the solid electrolyte, a multi-layered film, constructed by a high molecular film or a thin metal film, may be used. In particular, the moisture-proofing multi-layer film, made up of a heat-fused resin layer and a metal foil layer, is a highly promising candidate of the external sheathing material in that is helps realize a hermetically sealed structure by hot seal, and in that the multi-layer film itself has superior strength and air-tightness, while being lighter in weight, thinner in thickness and less expensive than a metallic casing.
However, if the above-mentioned film is used as the exterior material for the battery, and a low-boiling solvent is used as solvent for the electrolyte, the internal pressure in the battery tends to be increased with an increased vapor pressure of the solvent to produce swelling in case the battery is placed in a high temperature environment. Therefore, if a film is used as eternal material for the battery, a solvent needs to be selected taking the boiling point into account.
If the gel electrolyte is to be constructed, it is impossible to construct a gel unless the solvent for the electrolyte is compatible with the matrix polymer. For this reason, the solvent needs to be selected taking into account the compatibility with respect to the matrix polymer.
The low-boiling solvent, conventionally used in the lithium ion battery, such as dimethyl carbonate, ethyl methyl carbonate or diethyl carbonate, is high in solidifying point and low in viscosity and hence is highly effective to raise the ion conductivity of the electrolyte at lower temperatures. However, these solvents cannot be used as the solvent for the gel electrolyte used as the external sheathing material for the battery due to the constraint in compatibility or boiling point.
Thus, in the gel electrolyte, ion conductivity is generally lower than the solution electrolyte, due to limitations in the usable solvents, thus leading to an increased internal resistance of the batteries. In particular, in a frigid environment, such as xe2x88x9220xc2x0 C., the internal resistance is appreciably increased to render discharge almost impossible. That is, the low ion conductivity in the gel electrolyte frustrates attempts in improving the battery performance.
The above-mentioned problem can be solved by adding xcex3-butyrolactone GBL into a solvent. Since GBL is low in viscosity and in melting point, it has good ion conductivity and allows to cause large current to flow. GBL has an ion conductivity better than that of other high boiling solvents even at lower temperatures. Moreover, GBL has a higher dielectric constant and is able to dissolve an electrolyte salt to a high concentration.
Moreover, GBL is highly compatible with respect to polyvinylidene fluoride PVdF or a copolymer of PVdF and hexafluoro propylene (HFP), used as a matrix polymer of the gel electrolyte. Thus, GBL is a superior solvent if compatibility with respect to the matrix polymer for forming the gel electrolyte is also taken into account.
However, the lithium ion battery, employing GBL as a solvent for the electrolyte, is generally low in cyclic characteristics. This is felt to be ascribable to redox reaction GBL undergoes on the negative electrode. It has thus been difficult to fabricate a lithium ion secondary battery exhibiting satisfactory cyclic characteristics without impairing low temperature characteristics, load characteristics (large current characteristics) or stability of the gel electrolyte.
It is therefore an object of the present invention to provide a gel electrolyte and a gel electrolyte battery which improves ion conductivity of the solvent and which also is superior in cyclic characteristics.
In one aspect, the present invention provides a gel electrolyte including an electrolyte, a matrix polymer and a non-aqueous solvent. The non-aqueous solvent is a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC) and xcex3-butyrolactone (GBL). The non-aqueous solvent is of a weight composition in an area in a triangular phase diagram (EC, PC, GBL) surrounded by a point (70, 30, 0), a point (55, 15, 30), a point (15, 55, 30) and a point (30, 70, 0).
In the gel electrolyte according to the present invention, the solvent composition in the gel electrolyte is optimized, so that electrolyte decomposition is suppressed to improve ion conductivity and electrically conductivity and hence the gel electrolyte is optimum for use as an electrolyte for a battery.
In another aspect, the present invention provides a gel electrolyte battery including a positive electrode containing an active material for the positive electrode, a negative electrode containing an active material for the negative electrode and which is arranged facing the positive electrode, and a gel electrolyte arranged between the positive electrode and the negative electrode. The gel electrolyte includes an electrolyte, a matrix polymer and a non-aqueous solvent. The non-aqueous solvent is a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC) and xcex3-butyrolactone (GBL). The non-aqueous solvent is of a weight composition in an area in a triangular phase diagram (EC, PC, GBL) surrounded by a point (70, 30, 0), a point (55, 15, 30), a point (15, 55, 30) and a point (30, 70, 0).
In the gel electrolyte battery according to the present invention, , the solvent composition in the gel electrolyte is optimized, so that the gel electrolyte is improved in ion conductivity and electrically conductivity to render it possible to compromise variable battery characteristics, such as cyclic characteristics, low temperature characteristics or initial charging/discharging characteristics.
According to the present invention, a solid electrolyte in which electrolyte decomposition is suppressed to a minimum can be realized by optimizing the solvent composition in the electrolyte.
Moreover, with the use of the solid electrolyte of the present invention, a solid electrolyte may be realized which has improved cyclic characteristics and improved overall battery performance without impairing the initial charging/discharging efficiency, battery capacity, large current discharge or discharge under low temperature environment. The present solid electrolyte battery may be advantageously used as a power source for portable electronic equipments, such as portable telephone, video camera or notebook type personal computer.